BWRs have conventionally utilized active safety systems to control and mitigate accident events. Those events varied from small break to design basis accidents. Passive safety systems have been studied for use in simplified BWRs (SBWRs) because of their merits in reducing specialized maintenance and surveillance testing of the safety-related equipment, and in eliminating the need for AC power, thereby improving the reliability of essential safety system responses necessary for the control and mitigation of adverse effects produced by accidents. SBWRs can additionally be designed with certain passive safety features that provide more resistance to human error in accident control and mitigation.
The current SBWR designs utilize passive operational principles for the key safety systems employed to (a) provide emergency coolant injection, for assured core cooling over the design basis post-LOCA lifetime (specifically, 72 hr for these designs), and (b) provide assured containment heat removal over this same design basis accident duration. The decay heat removal accomplishes, by essentially passive means, the transfer of core decay heat (which manifests itself ultimately as hot steam inside the containment drywell) to the reactor building environs through the use of passive containment cooling (PCC) heat exchangers, as disclosed, for example, in U.S. Pat. No. 5,295,168.
The term "passive" as used to describe the actions of such safety systems, is defined to include systems which operate exclusively on stored energy, such as batteries, or pressurized gases, or chemical charges, or appropriately positioned tanks of water which can drain by gravity, to accomplish the essential safety function. The term "passive" further implies that no rotating or reciprocating machinery is used; valves, where used, are one-time change-of-position valves such as squib valves, or, in the case of check valves, are altogether unpowered insofar as their open/close state is concerned. Where water is used in such passive systems for flooding, quantities must be sufficient to accomplish all design goals over the prescribed accident period, i.e., 72 hr. Where water is used for decay heat rejection via the process of evaporation, quantities again must be sufficient to allow for boiloff of such pools without the pool drawdown uncovering the critical heat exchanger heat transfer surfaces (tubes) through which the heat is transferred via condensation of steam inside the tubes, and evaporation of a secondary water quantity (pool) external to these tubes, which secondary pool communicates via piping/ducting to the environs.
A typical SBWR reactor building arrangement has a plurality of PCC heat exchangers positioned in an interconnected series of pool chambers collectively referred to herein as the condenser pool. This pool requires a certain critical water inventory, i.e., the inventory for which the integrated design (pool plus positioned heat exchanger) is reckoned as boiloff volume. Before focusing on this aspect of water inventory management, a brief summary of the overall structure and operation of the current SBWR design is given below.
Referring to FIG. 1, the SBWR includes a reactor pressure vessel 10 containing a nuclear reactor fuel core 12 submerged in water 14. The fuel core heats the water to generate steam 14a which is discharged from the reactor pressure vessel through a main steam line 16 and used to power a steam turbine-generator for producing electrical power.
The reactor pressure vessel is surrounded by a containment vessel 18. The volume inside containment vessel 18 and outside reactor pressure vessel 10 is called the drywell 20. The containment vessel is a concrete structure having a steel liner and is designed to withstand elevated pressure inside the drywell. The drywell typically contains a noncondensable gas such as nitrogen.
In accordance with the conventional SBWR containment design, an annular suppression or wetwell pool 22 surrounds the reactor pressure vessel within the containment vessel. The suppression pool is partially filled with water 24 to define a wetwell airspace or plenum 26 thereabove. The suppression pool 22 serves various functions including being a heat sink in the event of certain accidents. For example, one type of accident designed for is a loss-of-coolant accident (LOCA) in which steam from the reactor pressure vessel 10 leaks into the drywell 20. Following the LOCA, the reactor is shut down but pressurized steam and residual decay heat continue to be generated for a certain time following the shutdown. Steam escaping into the drywell 20 is channeled into the suppression pool 22 through a multiplicity of (e.g., eight) vertical flow channels, each flow channel 27 having plurality of (e.g., three) horizontal vents 28. Steam channeled into the suppression pool 22 through the vents 28 carries with it portions of the drywell noncondensable gas 30. The steam is condensed and the noncondensable gas 30 is buoyed upwardly to the wetwell plenum 26, where it accumulates.
When the pressure in wetwell plenum 26 exceeds that in drywell 20, one or more vacuum breakers 36, which penetrate the wetwell wall, are opened to allow non-condensable gas 30 in the wetwell plenum 26 to vent to the drywell 20. The vacuum breakers 36 remain closed when the pressure in drywell 20 is equal to or greater than the pressure in the wetwell plenum 26.
The system further includes one or more gravity-driven cooling system (GDCS) pools 38 located above the suppression pool 22 within the containment vessel 18. The GDCS pool 38 is partially filled with water 42 to define a GDCS plenum 44 thereabove. The GDCS pool 38 is connected to an outlet line 46 having a valve 48 which is controlled by controller 40. The valve 48 is opened to allow GDCS water 42 to drain by gravity into pressure vessel 10 for cooling the core following a LOCA. Steam and noncondensable gas can be channeled directly into the GDCS plenum 44 from the drywell 20 via an inlet 50. An optional condenser or heat exchanger 72 may be provided for condensing steam channeled through inlet 50 following draining of the GDCS water 42 for drawing in additional steam and noncondensable gas.
The suppression pool 22 is disposed at an elevation which is above the core 12 and is connected to an outlet line 32 having a valve 34 which is controlled by a controller 40. The valve 34 is opened after an appropriate time delay from the opening of valve 48 to allow wetwell water 24 to also drain by gravity into the pressure vessel 10 for cooling the core following a LOCA.
In the SBWR design, a passive containment cooling system (PCCS) is provided for removing heat from the containment vessel 18 during a LOCA. A condenser pool 52, configured as a collection of subpools (not shown) interconnected so as to act as a single common large pool, is disposed above the containment vessel 18 and above the GDCS pool 38. The condenser pool 52 contains a plurality of PCC heat exchangers 54 (only one of which is shown in FIG. 1), also commonly referred to as PCC condensers, submerged in isolation water 56. The condenser pool 52 includes one or more vents 58 to atmosphere outside the containment for venting the airspace above the condenser pool water 56 for discharging heat therefrom upon use of the PCC heat exchanger 54.
The PCC heat exchanger 54 has an inlet line 60 in flow communication with the drywell 20 and an outlet line 62 joined to a collector chamber 64 from which a vent pipe 66 extends into the suppression pool 22 and a condensate return conduit 68 extends into the GDCS pool 38. The PCC heat exchanger 54 provides passive heat removal from the drywell 20 following the LOCA, with steam released into the drywell flowing through inlet 60 into the PCC heat exchanger wherein it is condensed. The noncondensable gas (e.g., nitrogen) within the drywell is carried by the steam into the PCC heat exchanger and must be separated from the steam to provide effective operation of the PCC heat exchanger. The collector chamber 64 separates the noncondensable gas from the condensate, with the separated noncondensable gas being vented into the suppression pool 22, and the condensate being channeled into the GDCS pool 38. A water trap or loop seal 70 is provided at the end of the condensate return conduit 68 in the GDCS pool 38 to restrict backflow of heated fluids from the GDCS pool 38 to the suppression pool 22 via the condensate return conduit 68, which would bypass PCC heat exchanger 54.
Accordingly, this system is configured to transport the noncondensable gas from the drywell 20 to the wet-well plenum 26 and then condense steam from the drywell in the PCC heat exchanger 54. The noncondensable gas will remain in the enclosed wetwell until the PCC heat exchanger 54 condenses steam faster than it is released from the reactor pressure vessel. When this occurs, the PCC heat exchanger lowers the drywell pressure below that of the wetwell, which causes the vacuum breakers 36 to open, thereby allowing noncondensable gas stored in the wetwell to return to the drywell.
As shown in greater detail in FIG. 2, the PCC heat exchanger 54 is a drum and tube heat exchanger comprising an upper drum 74 and a lower drum 76 connected via a multiplicity of vertical tubes 78. The PCC heat exchanger 54 is positioned within reactor building 80 in a chamber 52a of condenser pool 52. Pool chamber 52a is bounded by vertical walls 82 and 84, floor 86 and ceiling 88. The upper surface 90 of ceiling 88 is commonly the refueling floor of reactor building 80. A hatch 92 standing above the PCC heat exchanger 54 has a cover (not shown) which is removable to allow access to PCC heat exchanger 54 for servicing. During operation following a LOCA, as heat is conducted out of PCC heat exchanger 54, secondary steam formed in pool chamber 52a flows through airspace 94 and passes through moisture separator/dryer unit 96 and then through outlet piping 98 to reach the environs outside reactor building 80.
The allowable drawdown (i.e., boiloff) volume for pool chamber 52a has conventionally been taken to be that volume represented by all initial pool inventory located above the lower horizontal tangent to upper drum 74 of PCC heat exchanger 54 (diagrammed as "ELEV. A" in FIG. 2) up to the initial pool surface level. The effective pool inventory is amplified, in this ESBWR design, by providing a multiplicity of interconnected auxiliary pool chambers, where the interconnections are accommodated via piping and open-positioned valves. Auxiliary pool chamber 52b, shown in FIG. 2, is bounded by vertical walls 81 and 84, floor 86 and ceiling 88 and is connected to pool chamber 52a via piping 100 and valve 102. As is evident from FIG. 2, any preferential boiloff occurring to the water inventory in pool chamber 52a is passively replaced by drawdowns via gravity action in all interconnected pool chambers 52b, so that level remains essentially uniform throughout the entire interconnected pool chamber system, namely, condenser pool 52.
As is apparent from the foregoing description, a sizeable portion of condenser pool 52 is "underutilized", i.e., not given credit for boiloff The underutilized portion is the entire portion of condenser pool 52 standing below ELEV. A in FIG. 2. No other constructive use has yet been identified for this portion of the condenser pool 52, although it is recognized that some very modest credit may associate with a warmup of this water to boiling temperatures.
It would be desirable to minimize the required amounts of water in the condenser pool 52. This is because, among other reasons, such water represents a large mass which is located high in the reactor building 80, and which therefore represents a considerable design challenge to the reactor building structural designer in accommodating the resultant seismic loadings. Even more important, the requirements for adequate boiloff quantity in the condenser pool 52 means that quite some number of "auxiliary" pool chambers 52b must be provided, and this translates into considerable expanse of pool area, which in many cases sets the allowable minimum width and length of the reactor building. Furthermore, any approach which attempts to meet the water inventory need by increasing the depth of the condenser pool results in even greater overall plant costs.
Thus, there is a need for an economically advantageous passive means for constructively employing some selected amount of the heretofore "underutilized" water inventories in auxiliary pool chambers to increase the "credited" amount of heat removal during a LOCA via the PCC heat exchangers.